Methylated benzene oxidation



United States Patent Ofilice i 3330A l 3 Patented Apr. 1 7, l 9623,030,413 IVETHYLATED BENZENE OXIDATION Milton A. Taves, Wilmington,DeL, assignor to Hercules Powder Company, Wilmington, DeL, a corporationof Delaware Ntzzgfitrg'iigf 7Colngtgriauat ilon of application Ser. No.

ay his a lication Jul S61. No. 41,761 pp y 1960 3 Claims. (Cl. 260-524)This invention relates to an improved process for the oxidation ofaromatic hydrocarbons. In a specific aspect, this invention relates toan improved process for the oxidation of alkylated benzenes. In a morespecific aspect, this invention relates to an improved process for theoxidation of p-xylene to p-toluic acid.

In the air oxidation of aromatic hydrocarbons, such as alkylatedbenzenes, a variety of undesirable ester intermediates are formed. Forexample, in the air oxidation of p-xylene to p-toluic acid, esters suchas p-methylbenzyl, p-toluate and p-carboxybenzyl p-toluate are formed.Such esters are undesirable primarily because they lead to the formationof slimy oxidates with small crystal sizes that are exceedinglydifficult to filter. Furthermore, these esters represent a yield loss ifthey are not converted to p-toluic acid. When the oxidation is carriedout at atmospheric pressure, the esters can accumulate toconcentrations. as high as 40% and 50%. When the oxidation is carriedout at superatmospheric pressures, less esters are formed, butsufiicient esters are formed at superatmospheric pressures to presentproblems such as the necessity for disposing of the esters and loss ofxylene in the formation of the esters.

It has been found that the ester number of the oxidate resulting fromthe air oxidation of an alkylated benzene, such as p-xylene, can besubstantially reduced by carrying out the oxidation reaction in thepresence of water. The water presumably reduces the esters formed duringthe oxidation reaction by hydrolysis. However, it is possible that thepresence of the water alters the reaction in a manner more complex thansimple hydrolysis. Regardless of the manner in which the ester number ofthe oxidate is reduced, the presence or" the water has been found affectthe reaction mechanism in such a way that the amount of ester in theoxidate is substantially reduced or completely eliminated, and theattendant problems are similarly reduced or eliminated.

Example 1 Two thousand parts by weight of p-Xylene were charged to astainless steel autoclave along with 180 ppm. of cobalt as cobaltoustoluate. Air was passed through the p-xylene at a rate of 3 liters perkilogram per minute for a period of about hours at a temperature of 125C. and at atmospheric pressure. The oxidate from this reaction had anacid number of 205 and an ester number of 41.

Example 2 Two thousand parts by weight of p-xylene were charged to astainless steel autoclave along with 4 parts of cobaltous toluate. Airwas passed through the p-xylene at a rate of s.c.f. per kilogramp-xylene per hour for a period of about 2 hours at a temperature of 140C. and a pressure of 150 p.s.i.g. About 86.3 parts of water were formedduring the oxidation. The water of reaction that appeared in the off-gasfrom the reactor was removed from the system and was not returned to thereactor. The

oxidate from this reaction was found to have an acid number of 200 andan ester number of 22.5.

Example 3 At the conditions employed in Example 2, p-xylene was oxidizedto p-toluic acid, but water was retained in the reaction medium bycondensing it from the off-gas and returning it to the reactor. About88.6 parts of water were formed during the oxidation. The oxidate fromthis reaction had an acid number of 222.5 and an ester number of 1.7. Bycomparing the analyses of the oxidates from these two examples, it isapparent that the presence of the water substantially reduced the esternumber of the oxidate.

Example 1 demonstrates that, when the oxidation reaction is effected atconditions permitting substantially complete removal of water, theoxidate has a relatively high ester number. Example 2 indicates that byincreasing the reaction pressure from atmospheric to 150 p.s.i.g. it ispossible to reduce the ester content of the oxidate.

' Example 3 when compared with Example 2 demonstrates that at the samereaction conditions it is possible to reduce the ester number evenfurther by returning the water of reaction to the system.

Example 4 At the conditions employed in Example 1, a mixture of 1330parts by weight of p-xylene, 670 parts by weight of water, and 4.0 partsby weight of cobaltous acetate were contacted with air for a period of 3hours. At the end of this time, it was found that substantially nooxygen absorption had taken place indicating that there had beensubstantially no oxidation of the p-xylene. The presence of the water inthe autoclave made it diflicult, if not virtually impossible, to startthe oxidation at the conditions employed.

In practicing this invention, the oxidation of the alkylated benzene iscarried out at a temperature and pressure such that the oxidationreaction medium is in the liquid phase. The temperature can vary fromabout C. to about 250C. with a preferable temperature range being aboutto C. A temperature of at least 110 C. is essential in order for thereaction to proceed at a practical rate. The pressure is suitablyadjusted to maintain the oxidation medium in the liquid phase, andusually a superatmospheric pressure of from 15 to 400 p.s.i.g. andhigher is used. At low pressures the formation of undesired esterspresents a greater problem in such an oxidation, and this invention isparticularly useful at low pressures, for example, a superatmosphericpressure not in excess of 200 p.s.i.g., in minimizing the problemscaused by ester formation. However, the invention can be practiced athigh pressures, and when that is done the problems encountered byundesirable ester formation are also reduced. The invention can bepracticed by raising the oxidation pressure to a level at whichsufilcient water is retained in the system to reduce the ester contentof the oxidate substantially. However, lower pressures, for example, 15to 400 p.s.i.g. and preferably 50 to 175 p.s.i.g., are often desirableto reduce equipment and operating costs. At these lower pressures,sufiicient water is ordinarily not retained in the system to produce thedesired reduction of ester'content as demonstrated by Example 3 above..At these lower pressures, the invention is prac ticed by adding waterto the oxidation reaction either by returning water of reaction to theoxidation or by introducing water to the oxidizer from an outsidesource.

In carrying out the oxidation, a cobalt salt of an organic acidpreferably is employed. Such cobalt salts as cobalt toluate, cobaltnaphthenate, cobalt acetate, and cobalt salts of saturated aliphaticacids containing from about 6 to 12 carbon atoms can be used. The amountof catalyst that is employed to effect the oxidation is variable,

and generally from 10 to 400 parts per million of cobalt are present inthe oxidation reaction medium. However, it will be realized thatcatalyst concentrations outside this range and other metal catalyst thancobalt can be used when desired. Suitable catalysts for the om'dationreaction are those that are known for use in oxidation with gaseousoxygen. Salts of metals having more than one valence and selected fromthe group consisting of cobalt, manganese, iron and mercury can be used.

The hydrocarbons that are oxidized in accordance with this invention arethe alkylated benzenes. For example, toluene, the xylenes,ethylbenzene,propylbenzene, and the like, can be employed. The preferredhydrocarbons are of the dialkyl type and the alkyl groups usuallycontainno more than about 4 carbon atoms per alkyl group. Any of thexylenes can be oxidized in accordance with this invention, and it ispreferred to oxidize p-xylene to p-toluic acid. This acid is quiteuseful in the production of dimethyl terephthalate since it can becsterified to the monoester which, after another oxidation, can beesterified to the diester.

To effect the oxidation, an oxygen-containing gas is passed through theliquid reaction medium. Air is the preferred oxygen-containing gas.However, if desired, substantially pure oxygen as well asoxygen-enriched or oxygen-depleted air can be employed. However, in mostinstances, airwill be used as the oxidizing agent. It is usuallydesirable to employ the oxygen-containing gas at a rate such that theoff-gas from the reactor contains up to about 5% or oxygen by volume.

The amount of water that is necessary in the reaction medium isdependent upon the reaction conditions employed. At certain conditions,for example, at atmos pheric pressure, greater amounts of esters areformed than at super-atmospheric pressures because Water cannot readilybe retained in the reaction mixture at atmospheric pressure. Regardlessof the reaction conditions, the amount of water that is employed issufficient to substantially reduce the ester number of the oxidate. Ingeneral, the reaction medium contains a water concentration within therange of 0.5% to 50% by weight. Lesser amounts will reduce the esternumber to a lesser extent, but greater amounts can be used when desired.Usually an amount of water at least stoichiometrically equivalent to theesters formed during the oxidation is used. An amount in excess of thestoichiometric equivalent can be used to assure substantially completeremoval of the esters. It is also frequently advantageous to employwater in excess of the stoichiometric equivalent to provide a method foreither controlling or aiding in the control of the reaction temperature.The'oxidation is exothermic, and, when an excess of water is employed,exothermic heat of reaction can be dissipated by evaporation of waterintroduced to the system.

The water that is essential for practicing this invention can be aproduct of the oxidation reaction. One method of retaining sufficientwater in the reaction medium involves the use of pressures suflicientlyhigh to permit such retention. However, the method necessitates the useof costly high pressure equipment and procedures. A method for use atlower pressures is similar to that described in Example 3 above. In thismethod the reaction temperature aud pressure are such that part of thewater from the reaction medium leaves the oxidation reactor with theofi-gas. The off-gas is cooled and liquefiable components, includingwater of reaction, are condensed and returned to the reactor. Ifdesired, the water may be separated from the condensate before beingrecycled to the oxidizer. In another procedure for use .4 at lowerpressures, Water from an outside source is injected into the reactionmedium. This injected water has the effect of reducing the ester numberof the oxidate in the same manner in which water of reaction reduces theester number. However, when water from an outside source is introducedto the reactor operating at the preferred conditions of temperature andpressure, it should not be introduced to the reactor until the oxidationhas been initiated, since the presence of the water in the reactor makesdifficult the initiation of the oxidation. This fact is demonstrated byExample 4. When conditions more severe than those of Example 4 areemployed, for example, at a pressure of 350 p.s.i.g. and higher and atemperature of 200 C. and higher, the oxidation reaction can beinitiated in the presence of water.

When this invention is practiced, the ester number of the oxidatecontaining the desired organic acids can be maintained at a level below10 and preferably below 5. In fact, the invention can be practiced tomaintain the ester number of the oxidate at substantially zero.

V This invention is readily adaptable to continuous or batch operation.In either case, after the desired oxidation been effected, oxidate isremoved from the reactor and the acid or'acids are recovered,for'example, by a crystallization andfiltration procedure.

The following procedure was used to determine acid number. A 4 to 5 g.oxidate sample was dissolved in 50 ml. methanol or ethanol, and thesolution was titrated to neutrality using 0.1 N NaQH and phenolphthaleinindicator. The acid number of the sample was calculated from theformula:" i

Acid Ml. allraliXNX 56.1

. Grams sample The saponification number was determined by refluxing aseparate sarnple'of the oxidate with a known amount of 0.8-0.9 N aqueousKOH in a 1:1 aqueous alcohol solution and determining the amount ofalkali consumed by titrating the amount remaining with 0.5 N'I-ICl. Thesaponification number was calculated from the formula:

Sapon. NO.=W

- Grams sample where S is the ml. HCl required to titrate the sample andB is the ml. I-ICl required to titrate a blank to which no sample wasadded. The ester number is the saponification number minus the acidnumber.

Modifications and advantages of this invention will be readily apparentto those skilled in the art from the above disclosure.

This application is a continuation of my copending application SerialNo. 428,368, filed May 7, 1954, now abandoned. What I claim and desireto protect by Letters Patent 1s:

1. In a process for oxidizing a methylated benzene having not more thantwo methyl groups in liquid phase with an oxygen-containing gas in thepresence of an oxidation catalyst to the corresponding acid at atemperature of -250 C. and at a pressure not exceeding 400 p.s.i., theimprovement of initiating the oxidation under substantially anhydrousconditions, condensing the water of reaction, and returning said waterof reaction to the reaction mixture as the reaction proceeds in theamount to provide a water concentration of 0.5 to.50% by weight wherebythe ester number of the reaction mixture is substantially reduced.

2. In a process for oxidizing a methylated benzene having not more thantwo methyl groups in liquid phase with an oxygen-containing gas in thepresence of an oxidation catalyst to the corresponding acid at atemperature of 80-250 C. and at a pressure not exceeding 400 p.s.i., theimprovement of initiating the oxidation under substantially anhydrousconditions, withdrawing a 6 substantial amount of water resulting fromthe oxidation catalyst is a cobalt salt of an organic acid, thetemperareaction in the gaseous phase from said reaction, and ture is 125to 175 C., and the pressure is a superatinjecting Water from an outsidesource into the reaction mospheric pressure not above 200 p.s.i.g.mixture as the reaction proceeds in an amount such that the reactionmixture contains a water concentration With- 5 References Cited in thefile of this P in the range of 0.5 to 50% by weight and sufiicient toreduce substantially the ester number of said reaction UNITED STATESPATENTS mixture. 1,815,985 Pansegrau July 28, 1931 3, The process ofclaim 1 wherein the methylated ben- 2,680,757 Himel June 1954 zene isp-xylene, the oxygen-containing gas is air, the 10 2,727,921 Taves 20,1955

1. IN A PROCESS FOR OXIDIZING A METHYLATED BENZENE HAVING NOT MORE THANTWO METHYL GROUPS IN LIQUID PHASE WITH AN OXYGEN-CONTAINING GAS IN THEPRESENCE OF AN OXIDATION CATALYST TO THE CORRESPONDING ACID AT ATEMPERATURE OF 80-250*C. AND AT A PRESSURE NOT EXCEEDING 400 P.S.I., THEIMPROVEMENT OF INITIATING THE OXIDATION UNDER SUBSTANTIALLY ANHYDROUSCONDITIONS, CONDENSING THE WATER OF REACTION, AND RETURNING SAID WATEROF REACTION TO THE REACTION MIXTURE AS THE REACTION PROCEEDS IN THEAMOUNT TO PROVIDE A WATER CONCENTRATION OF 0.5 TO 50% BY WEIGHT WHEREBYTHE ESTER NUMBER OF THE REACTION MIXTURE IS SUBSTANTIALLY REDUCED.